The large tumor antigen (T antigen) of simian virus 40 (SV40) is a multifunctional protein that is required for several steps in productive viral infection. Thus, T antigen function(s) are required for viral DNA replication, the negative control of early- region mRNA synthesis, the activation of late-region transcription and at least one other, as yet unidentified step involving late gene expression or assembly. In addition to its role in production infection, T antigen acts as an oncogene in many cell-types, inducing the morphological transformation of cells in culture and tumors in animals. Studies from a number of laboratories have shown that the 708 amino acid protein consists of several functional domains, each of which can function somewhat independently of the rest of the polypeptide. In addition T antigen occurs in several oligomeric forms and subclasses that differ in their type and degree of posttranslational modification. One approach to learning to nature of each T antigen activity and its role in viral development and transformation is to isolate mutants that selectively affect individual functions. Towards this end we have been characterizing a collection of deletion and amino acid substitution mutants that alter T antigen structure. We propose to continue this approach with special emphasis on discerning those activities to T antigen that play a role in transformation. Thus, we plan to: (1) map the minimal sequences required for the transformation of different cell types and relate this activity to other T antigen functions; (2) assess the ability of this transforming region(s) to cooperate with other oncogenes and induce tumors in transgenic mice; (3) map the minimal sequences that induce transcriptional trans activation and assess the relation of this activity to transformation; and (4) continue to characterize selected mutants and recombinant viruses.